1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to an optical link for exchanging Radio Frequency (RF) signals between a remote station and a base station in a Radio over Fiber (RoF) system using wireless the uplink/downlink signal transmission of a Time Division Duplexing (TDD) mobile communication system and the configuration of the RoF system.
2. Description of the Related Art
With the development of the wireless communication industry, various wireless communication schemes have been proposed. Thus, there is an added benefit in proposing a mobile communication network that can support a new/different wireless communication scheme other than an existing wireless communication scheme, and in addition, support the existing wireless communication scheme. In other words, a wireless environment where mobile communication networks supporting different wireless communication schemes co-exist is expected to emerge.
In such a wireless environment, users having mobility need to select a suitable wireless communication scheme for wireless conditions.
A 3rd Generation (3G) wireless communication system has evolved to perform high-speed and high-capacity data communication, in addition to conventional voice communication.
Moreover, the 3G wireless communication system has been discussed by the 3rd Generation Partnership Project (3GPP) that is the European asynchronous standardization organization and the 3GPP2 that is the U.S synchronous standardization organization. A representative scheme under discussion in the 3GPP is a Wideband Code Division Multiple Access (WCDMA) scheme and a representative scheme under discussion in the 3GPP2 is a Code Division Multiple Access (CDMA) scheme.
An existing wireless mobile communication system typically uses an optical relay station in order to enlarge its cell coverage and remove an electric wave shadow region. In particular, in an underground or the inside of a building which electric waves cannot reach, a wired optical relay station is widely used. The optical relay station is configured to be suitable for the existing wireless mobile communication system using a Frequency Division Duplexing (FDD) scheme such as CDMA or WCDMA. However, a recently emerging new wireless mobile communication system such as Mobile Worldwide Interoperability for Microwave Access (WiMAX), or Wireless Broadband Internet (Wibro), makes use of a Time Division Duplexing (TDD) scheme wherein the same frequency is used for both uplink signal transmission and downlink signal transmission. The uplink and downlink transmissions are distinguished by time. Therefore, a wired/wireless relaying technique suitable for the new wireless mobile communication system is required.
In a communication system using the TDD scheme, a base station may allocate, for example, all available time slots, or some available time slots to a particular terminal. Thus, uplink/downlink transmission capacity is relatively free to change the allocations of slots, thereby allowing asymmetric communication by means of variable allocation of time slots. In addition, the channel characteristics of uplink/downlink radio signals are the same. For these reasons, the TDD scheme is recognized as being suitable for a next generation wireless mobile communication system using multiple antennas.
However, a problem in the TDD scheme is that as the radius of a cell increases, there is a degradation in transmission efficiency as a guard time interval between transmission/reception time slots increases due to a round trip delay. As a result, the TDD scheme undergoes degradation in transmission efficiency in a wireless communication system such as a macro cell having a large radius.
In contrast, a wireless mobile communication system using the FDD scheme does not experience a time delay in transmission or reception because of using separate frequency bands for transmission and reception. Since there is no a round trip delay caused by a time delay, the FDD scheme is suitable for a cell having a large radius such as a macro cell. However, the FDD scheme is not suitable for use as a duplexing technique for asymmetric transmission because a transmission/reception frequency and is fixed.
With regard to mobile communications, the 3G mobile communication system and next generation wireless mobile communication systems such as Mobile WiMAX and International Mobile Telecommunication (IMT)-Advanced systems aim to support both a voice service and multimedia services having various traffic characteristics such as broadcasting and real-time video conferencing. In order to efficiently provide the services having variously desired characteristics, there is a need for a duplexing scheme considering asymmetry and continuity of uplink and downlink transmissions according to the characteristics of the services.
At the present time, the CDMA or WCDMA system uses the FDD scheme and a Global System for Mobile Communication (GSM), a Wireless Local Area Network (WLAN) system, and the Mobile WiMAX system use the TDD scheme. Since two of the resources that can be provided to a plurality of wireless communication service subscribers are time and frequency according to space, there have been studies conducted with regard to a method and system for appropriately allocating the two resources, (i.e., time and frequency) according to the needs of a wireless channel condition such as a hybrid duplexing technique, in addition to the two existing duplexing schemes TDD and FDD.
FIG. 1 illustrates the structure of an RF relay station used to increase service capacity and enlarge service coverage in a wireless relay system for a conventional TDD wireless communication service.
Referring to FIG. 1, in order to provide a downlink signal transmission, link antenna 102 of the RF relay station receives an RF signal from a base station (not shown) and a band corresponding to the bandwidth of the RF signal is passed using a filter. The RF signal received through a circulator is amplified by a Low Noise Amplifier (LNA) 104. The frequency band of the RF signal from the LNA 104 is adjusted by a Gain Block (GB) 108. The RF signal whose frequency band has been adjusted is amplified by a High Power Amplifier (HPA) 112 and then passes through the filter through the circulator. The RF signal is then transmitted to a mobile terminal via a service antenna 114.
For an uplink transmission, the processing direction is opposite to that of downlink transmission described herein above. In other words, an RF signal is received from a mobile terminal (not shown) via the service antenna 114 of the RF relay station and a band corresponding to the bandwidth of the RF signal is passed using the filter The RF signal received through the circulator is amplified by the LNA 106. The frequency band of the RF signal from the LNA 106 is adjusted by the GB 108. The RF signal, whose frequency band has been adjusted is amplified by the HPA 112, then passes through the filter through the circulator/coupler. The RF signal is then transmitted to the mobile terminal via the link antenna 102.
However, in conventional techniques including that illustrated in FIG. 1, a GB can be shared for both uplink and downlink as a middle part of relatively low importance of the relay station. However, sharing of an LNA, which is a component of relatively high importance of the relay station, for both uplink and downlink communication is physically difficult to achieve. One reason that there is difficulty in sharing an LNA is that an HPA which finally amplifies uplink and downlink signals is typically connected to two antennas. Moreover, the sharing of the LNA for amplification of both uplink and downlink signals prevents the LNA from performing its essential function of maintaining low-noise amplification characteristics, which thereby degrades system performance.